Along with recent improvements in overall capabilities of personal computers, it has become feasible for the personal computer to process AV (audio-visual) information such as music and movie images, for example.
Since the data volume of AV information is considerably large, optical recording media have been attracting much attention as a viable means for storing such information data, as exemplified by CD (compact disc) and DVD (digital versatile disc) which are capable of recording the data about seven times as large as CD. Also, optical disc systems with lowering costs have been in wide spread use as the apparatuses accessible to those recording media.
An optical recording medium is formed in general with at least a recording surface provided thereon with tracks (or pregrooves) formed continuously in the shape of a spiral or concentric circle.
In addition, two regions are formed on the recording surface of the medium, respectively called mark (pit) and space regions each having light reflectivity different with each other, such that information data is recorded by the combination of these two regions each having suitable length and arrangement on the recording surface.
An optical disc apparatus is configured in general to perform record or erase of information data by a light spot incident on the surface of the recording medium, and data readout by a light beam reflected back (return beam) from the surface of the medium.
In order to carry out these steps the optical disc apparatus incorporates an optical pickup unit for emanating laser light beams and for receiving light beams reflected back from the surface of the medium.
The optical pickup unit includes several components such as an objective, an optics system for guiding laser beams emanated from a light source to the surface of the recording medium and return beams to a predetermined receiving location, and a light detecting element placed at the receiving location.
The detecting element is adapted to output not only readout information of the data recorded on the recording surface but also the signals containing the information (including servo-information) necessary for controlling the position of the optical pickup unit itself and the objective (or objective lens).
In order for information data to be recorded at proper locations on the media surface, and for the data recorded at specified locations on the surface to be readout, light spots have to be formed precisely at aiming points, and the points have to be detected precisely. It is important therefore to properly detect the points of the light spots on the recording surface, in which various methods for achieving the detection have been disclosed utilizing return beams from the surface of the recording medium.
In general, as the signal noted earlier which contains information regarding the point for forming the light spots in the direction perpendicular to recording tracks (which is hereinafter referred to as “tracking direction”) and track-error signals are utilized.
In the differential push-pull method (DPP method), for example, the light beams emanated from light source are divided into one main beam and two sub-beams. These light beams are then guided to be incident on recording media surface such that the sub-beams (SP2 and SP3) are each shifted by one-half track pitch (Tp/2) in the tracking direction Dtr with respect to the main beam (SP1) as shown in FIG. 10.
Return beams of the main and substrate-beams are then detected by three split-detectors respectively to output push-pull signals. Subsequently, track-error signals are obtained from the difference between the sum of sub-beam push-pull signals (SP2 and SP3) and the main beam push-pull signals.
Based on the track-error signals thus detected, servo-control in optical disc system is carried out so light spots are formed at the proper location on the media surface.
In addition, a variety of methods and apparatuses have been proposed for detecting the location of the light spots with high precision (Japanese Laid-Open Patent Applications No. 10-162383 and 2001-250250, for example).
The optical discs have been supplied in recent years by an increasing number of manufacturers along with rapid increase of users.
Along with this trend, however, the quality of optical discs supplied to general users has become an issue, in that some of the discs supplied in practice are of low quality with uneven track pitch for example, or sub-standard quality with a large fluctuation in track pitch depending on the type of disc.
Since signal characteristics of track-error signals fluctuate considerably for the discs of low or sub-standard quality, a possibility that light spots cannot be formed precisely at aiming points even after using apparatuses having high precision recording capabilities (as disclosed in the abovementioned '383 and '250 applications) is encountered.
In such case, users may not be aware of the difficulty in carrying out proper data recording, which is caused by low quality discs purchased previously. It is desirable therefore to provide an optical disc apparatus capable of accessing information, which are low in quality to a certain extent, or of sub-standard quality.